The Outer Planets and Pluto 7: Touring Our Solar System

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Features of Saturn

In 1980 and 1981, flyby missions of the Voyager 1 and Voyager 2 spacecraft came within 100,000 kilometers of Saturn. More information was gained in a few days than had been acquired since Galileo first viewed this elegant planet. More recently, observations from ground-based telescopes, the Hubble Space Telescope, and Cassini have added to our knowledge of Saturn's ring and moon system. When the positions of Earth and Saturn allowed the rings to be viewed edge-on—thereby reducing the glare from the main rings—Saturn's faintest rings and satellites became visible.

Jupiter: Giant Among Planets

Jupiter is only 1/800 as massive as the sun. Still, it is the largest planet by far. Jupiter has a mass that is 2 1/2 times greater than the mass of all the other planets and moons combined. In fact, had Jupiter been about 10 times larger, it would have evolved into a small star. Jupiter rotates more rapidly than any other planet, completing one rotation in slightly less than 10 Earth-hours.

Jupiter's Atmosphere

Jupiter's hydrogen-helium atmosphere is very active. It contains small amounts of methane, ammonia, water, and sulfur compounds. The wind systems generate the light- and dark-colored bands that encircle this giant. Unlike the winds on Earth, which are driven by solar energy, Jupiter itself gives off nearly twice as much heat as it receives from the sun. Thus, the interior heat from Jupiter produces huge convection currents in the atmosphere.

How does Jupiter's mass compare with that of all other planets and moons combined?

Jupiter's mass is 2 1/2 times greater than that of all other planets and moons combined.

Jupiter's Rings

Jupiter's ring system was one of the most unexpected discoveries made by Voyager 1. By analyzing how these rings scatter light, researchers concluded that the rings are composed of fine, dark particles, similar in size to smoke particles. The faint nature of the rings also indicates that these minute fragments are widely dispersed. The particles are thought to be fragments blasted by meteorite impacts from the surfaces of Metis and Adrastea, two small moons of Jupiter.

Jupiter's Moons

Jupiter's satellite system, consisting of 63 moons discovered so far, resembles a miniature solar system. The four largest moons, Io, Europa, Ganymede, and Callisto, were discovered by Galileo in 1610. Each of the four Galilean satellites is a unique geological world. The innermost of the Galilean moons, Io, is one of four known volcanically active bodies in our solar system.

Which planet once had an Earth-size blemish called the Great Dark Spot?

Neptune

Neptune: The Windy Planet

Neptune has a dynamic atmosphere, much like those of Jupiter and Saturn. Winds exceeding 1,000 kilometers per hour encircle Neptune, making it one of the windiest places in the solar system. It also had an Earth-size blemish called the Great Dark Spot that was reminiscent of Jupiter's Great Red Spot. The Great Dark Spot was assumed to be a large rotating storm. About five years after the Great Dark Spot was discovered, it vanished, only to be replaced by another dark spot in the planet's northern hemisphere, which also vanished within a few years.

Neptune: The Windy Planet 2

Neptune has many surprising features. Perhaps most surprising are the cirrus-like clouds that occupy a layer about 50 kilometers above the main cloud deck. The clouds are most likely frozen methane. Voyager images revealed that the bluish planet also has a ring system. Neptune has 13 known moons. Triton, Neptune's largest moon, is nearly the size of Earth's moon. Triton is the only large moon in the solar system that exhibits retrograde motion. This motion indicates that Triton formed independently of Neptune and was gravitationally captured. Triton also has the lowest surface temperature yet measured on any solar body in the solar system at -200°C. Its atmosphere is mostly nitrogen with a little methane. Despite low surface temperatures, Triton displays volcanic-like activity.

Pluto: Dwarf Planet 2

Pluto is the most well known of the dwarf planets. However, it is neither the largest nor the first to be discovered. The dwarf planet Ceres, which is in the asteroid belt, was discovered in 1801. Ceres was originally considered a new planet. Other objects discovered in similar orbits led to it and other objects being classified as asteroids, and now, a dwarf planet. Just another example of how science changes as new observations and data become available. The dwarf planet Eris, discovered in 2003, is slightly larger than Pluto. Along with Eris, several others, including Makemake and Haumea, are known to orbit the sun in the icy regions beyond Neptune. In honor of Pluto, the IAU has designated these trans-Neptunian dwarf planets plutoids. Plutoids are dwarf planets that have average orbits beyond the orbit of Neptune.

Saturn: The Elegant Planet

Requiring 29.46 Earth-years to make one revolution, Saturn is almost twice as far from the sun as Jupiter. However, its atmosphere, composition, and internal structure are thought to be remarkably similar to Jupiter's. The most prominent feature of Saturn is its system of rings. In 1610, Galileo used a primitive telescope and first saw the structures that were later found to be the rings. They appeared as two small bodies adjacent to the planet. Their ring nature was explained 50 years later by the Dutch astronomer Christian Huygens.

Which best describes Saturn's moons as scientists understand them today? (Note: The count of moons excludes the "moonlets" that make up Saturn's rings.)

Saturn has 56 moons, one of which is bigger than Mercury.

Saturn's Moons

Saturn's satellite system consists of 56 moons. Titan is the largest moon and is bigger than Mercury. It is covered with rivers and oceans of liquid hydrocarbons. Titan and Neptune's Triton are the only moons in the solar system known to have substantial atmospheres. Because of its dense gaseous cover, the atmospheric pressure at Titan's surface is about 1.5 times that at Earth's surface. Another moon, Enceladus, is one of four known volcanically active bodies in our solar system. In 2006, the Cassini space probe discovered liquid water geysers in the moon's south polar region.

Jupiter's Moons 2

The other volcanically active bodies are Earth, Saturn's moon Enceladus, and Neptune's moon Triton. The heat source for volcanic activity on Io is thought to be tidal energy generated by a relentless "tug of war" between Jupiter and the other Galilean moons. The gravitational power of Jupiter and nearby moons pulls and pushes on Io's tidal bulge as its orbit takes it alternately closer to and farther from Jupiter. This gravitational flexing of Io is transformed into frictional heat energy and results in Io's volcanic eruptions.

Pluto: Dwarf Planet

Until 2006, Pluto was considered to be one of the nine planets. But in August of 2006, the International Astronomical Union (IAU) redefined the word "planet" in a way that excluded Pluto. Pluto is not considered a planet because it shares its orbit with smaller objects. Because Pluto was no longer a planet, the IAU also created a new term to describe it. A dwarf planet is a round object that orbits the sun but has not cleared the neighborhood around its orbit. Unlike irregularly shaped asteroids, a dwarf planet is massive enough to create a gravitational force that pulls it into a round shape. However, a dwarf planet's gravity is too weak to pull in smaller nearby bodies. Therefore, a dwarf planet orbits in a zone along with other small solar system bodies. Astronomers estimate that there may be as many as 200 dwarf planets in the solar system.

Saturn's Rings

Until the discovery that Jupiter, Uranus, and Neptune also have ring systems, this phenomenon was thought to be unique to Saturn. Although the four known ring systems differ in detail, they share many attributes. They all consist of multiple concentric rings separated by gaps of various widths. In addition, each ring is composed of individual particles—"moonlets" of ice and rock—that circle the planet while regularly impacting one another. Most rings fall into one of two categories based on particle density. Saturn's main rings and the bright rings of Uranus are tightly packed and contain "moonlets" that range in size from a few centimeters to several meters. These particles are thought to collide frequently as they orbit the parent planet.

Jupiter: Giant Among Planets 2

When viewed through a telescope or binoculars, Jupiter appears to be covered with alternating bands of multicolored clouds that run parallel to its equator. The most striking feature is the Great Red Spot in the southern hemisphere. The Great Red Spot was first discovered more than three centuries ago by two astronomers, Giovanni Cassini (for whom the space probe was named) and Robert Hooke. When Pioneer 11 moved within 42,000 kilometers of Jupiter's cloud tops, images from the orbiter indicated that the Great Red Spot is a cyclonic storm.

What makes up the large, bright rings of Saturn?

a dense, thin set of "moonlets," or orbiting particles of ice or rock

Which best describes Jupiter?

a huge, massive planet with more than 60 moons

Uranus is unique among the planets because of its _____.

axis of rotation

Which is a characteristic feature of Neptune?

extremely fast winds

Pluto was once classified as the ninth planet. In 2006, Pluto was reclassified as a dwarf planet because it _____.

has not cleared the neighborhood around its orbit

Which best describes the mass of Jupiter?

much less massive than the sun, but more than twice as massive as all other planets and moons combined

Which best describes the rotational motion of Uranus as it orbits the sun?

rolling like a ball

Features of Saturn 2

1. Saturn's atmosphere is very active, with winds roaring at up to 1,500 kilometers per hour. 2. Large cyclonic "storms" similar to Jupiter's Great Red Spot, although smaller, occur in Saturn's atmosphere. 3. Eleven additional moons were discovered. 4. The rings of Saturn were found to be more complex than expected.

Uranus: The Sideways Planet 2

A surprise discovery in 1977 revealed that Uranus has a ring system. This find occurred as Uranus passed in front of a distant star and blocked its view. Observers saw the star "wink" briefly both before and after Uranus passed by. Later studies indicate that Uranus has at least nine distinct ring belts. The five largest moons of Uranus show varied terrain. Some of the moons have long, deep canyons and linear scars, whereas others possess large, smooth areas on otherwise crater-riddled surfaces. Miranda, the innermost of the five largest moons, has a greater variety of landforms than any solar body yet examined in the solar system.

Uranus: The Sideways Planet

A unique feature of Uranus is that it rotates "on its side." Instead of being generally perpendicular to the plane of its orbit like the other planets, Uranus's axis of rotation lies nearly parallel with the plane of its orbit. Its rotational motion, therefore, has the appearance of rolling, rather than the top-like spinning of the other planets. Uranus's spin may have been altered by a giant impact.

Pluto: Dwarf Planet 3

All dwarf planets likely contain a mixture of rock and ice, and they can be found in very different parts of the solar system. Ceres, for example, orbits the sun between Mars and Jupiter while the plutoids are found in the icy regions past Neptune. Pluto is slightly unusual in that it has a moon, Charon, which is more than half its size and may be considered a dwarf planet on its own.

Structure of Jupiter

Although Jupiter is called a gas giant, it is not simply a ball of gas. At 1,000 kilometers below the clouds, the pressure is great enough to compress hydrogen gas into a liquid. Consequently, Jupiter is thought to be a gigantic ocean of liquid hydrogen. Less than halfway into Jupiter's interior, extreme pressures cause the liquid hydrogen to turn into liquid metallic hydrogen. Jupiter is also believed to have a rocky and metallic central core.

Pluto is considered a dwarf planet, a category that also includes _____.

Ceres, located in the asteroid belt

Saturn's Rings 2

Despite the fact that Saturn's dense rings stretch across several hundred kilometers, they are very thin, perhaps less than 100 meters from top to bottom. At the other extreme, the faintest rings, such as Jupiter's ring system and Saturn's outermost rings, are composed of very fine particles that are widely dispersed. In addition to having very low particle densities, these rings tend to be thicker than Saturn's bright rings.